Many containers used to hold food products have a peelable lid which is easily detached prior to first use of the product by a consumer. Such peelable lids act both to seal the dispensing aperture of the container as well as to provide a tamper evidence indicator. A peelable lid provides an easy method of opening a container without the need for separate tools, such as scissors or can openers.
Containers with peelable lids can be made from a variety of different materials including glass, cardboard, plastic and metal. Cardboard containers may be suitable for holding food products in some situations; however, in certain markets, for example countries with relatively hot and/or humid climates, cardboard containers may not be appropriate. Susceptibility to attack by pests (e.g. mice and rats) and their relative lack of strength may also make cardboard containers unsuitable for many uses. Plastic and glass containers are also prone to some of the same problems. Metal containers or “cans” address many of these issues.
Cans for use in specialised market segments, for example cans used in the infant formula market, are required to satisfy stringent safety standards. This can be a challenge when producing a metal can with a peelable, typically foil, lid. For example, it is required that a can used to hold infant formula powder maintain its hermetic seal with a peelable lid even when stored in excess of 3 months at high temperatures, such as 45° C., and with a pressure difference of 700 mbar (70 kPa) between the inside and outside of the can.
Typically, a hermetic seal between a peelable foil lid and an appropriately configured inwardly directed flange or lip of a metal can is made by first heating the sealing surface of the flange or lip. Either the sealing surface or an opposed sealing surface of the lid is coated with a bonding material, typically either a lacquer or a polymer. The foil lid is applied to the flange and a seal is achieved by applying a combination of heat and pressure (applied to the lid in situ).
Where the filled, sealed can is to be subsequently processed to cook or otherwise heat the contents, a polypropylene coating may be provided on the interior surface of the can and on the lidding material such that these “weld together” to form a hermetic seal. Seals provided in this way are able to withstand the rigours of processing, such as a combination of high temperature (typically 120° C. or more) and pressure acting on the lid.
Heating of the flange may be achieved using conduction heating or induction heating. In the case of conduction heating, heat is transferred to the metal can via direct contact with the can. In the case of induction heating, a high frequency alternating current is passed through an electromagnetic induction coil to produce an electromagnetic field. The coil is placed around the outside of the can such that the can, and in particular the area surrounding the flange, is situated within the axially induced electromagnetic field. The resulting eddy currents created in the flange and surrounding can area give rise to a rapid heating of the flange. Induction heating is generally advantageous as compared with conduction heating as heating times with the former are faster, and direct contact with the can is not required (such that various can shapes can be used with the same induction heating set up).
For some uses, it is desirable to provide the foil lid at a position part-way along the can body, thus separating the can body into two compartments. The (hermetically sealed) compartment beneath the foil lid is used to contain the food product, whilst the compartment above the lid may contain some other item, e.g. a plastic spoon. The top opening of the can may be closed with a plastic lid or the like. Particularly for cans of this configuration, the known heating schemes can cause noticeable damage to the exterior of the can due to the high temperatures that the exterior must be heated to, in order to raise the flange sealing surface to the temperature required to achieve a seal. Typically, as the flange will cool slightly between initial heating and application of the foil lid, the sealing surface must be heated significantly above the setting temperature. In addition, as heat is conducted from the exterior surface of the can to the flange, that exterior surface must in turn be heated to an excessive temperature. For a bonding material that operates at around 160° C., the flange needs to be heated to 200° C., during which the exterior surface can reach a temperature of 280° C. The temperature required at the exterior surface is such that tin reflow may occur, resulting in visible marking (i.e. discoloration) to the exterior surface.
One possible solution is to use a bonding material which operates at a lower temperature than those that have typically been used for this purpose, e.g. a bonding material that sets at 90° C., so that it is not necessary to excessively heat the exterior surface of the can. However, this would render the cans unsuitable for sale in markets with hot climates.